This code sample is straight from MSDN, but it is so often used, I wanted to make sure I kept it handy:
// Add a note with a document attachment to the contact's record.
var attachment = File.OpenRead("capture.png");
var data = new byte[attachment.Length];
attachment.Read(data, 0, (int)attachment.Length);
var note = new Xrm.annotation()
subject = "Note subject...",
notetext = "Note Details....",
Contact_Annotation_id = contact.contactid,
objecttypecode = "contact",
documentbody = Convert.ToBase64String(data),
filename = Path.GetFileName(attachment.Name),
mimetype = "image/png"